In the context of fuel tanks, “inerting” may refer to the process of making ullage in a fuel tank non-flammable. The Federal Aviation Administration (FAA) reconsidered aircraft fuel tank safety after a series of fuel tank explosions between 1990 and 2001. The National Transport Safety Board (NTSB) added “Explosive Mixture in Fuel tanks in Transport Category Aircrafts” as the number one item in its 1997 “Most Wanted” Transportation Safety list.
Some known fuel tanks have a region containing liquid fuel and an ullage region that often contains evaporated fuel (i.e., fuel vapor). With the presence of air, the mixture may exhibit a fuel-air ratio within the ullage and ignition may occur when fuel-air ratio in the ullage lies within a certain range. The lower flammability limit is defined as a threshold below which the fuel-air ratio is too lean to ignite. Similarly, the upper flammability limit is defined as the threshold above which the fuel-air ratio is too rich to ignite. The fuel-air ratios between the lower flammability limit and the upper flammability limit are termed flammable.
At temperatures less than 100° F. at sea level, the ullage fuel-air ratio for Jet A fuel generally lies below the lower flammability limit. However, known conditions exist that may result in exceeding the lower flammability limit. One example includes a rapid reduction in tank ullage pressure after takeoff, such as when the aircraft reaches a high altitude in a short time before fuel-tank temperature substantially decreases.
FAA regulations require that new and in-service transport aircraft include systems for enhancing the safety of aircraft fuel tanks. For protection against fire/explosion in the fuel tank ullage, several previous methods have been used. One method is referred to as ullage inerting, and involves pumping an inert gas, such as nitrogen, into the ullage, to reduce the oxygen concentration therein. The nitrogen can be obtained from cryogenic storage bottles on board the aircraft, or from an Onboard Inert Gas Generator System (OBIGGS). OBIGGS is used in many commercial and cargo airplanes and some high performance military aircraft. Unfortunately, ullage inerting systems are expensive, complex, and increase the weight of the aircraft. They also add a logistics penalty.
Another method that has been used is filling the fuel tank with flexible open cell polyurethane foam. Due to its limited life and stability, several changes of the foam are required during the life of the aircraft, increasing maintenance costs. These foams also contribute to weight and volume penalties, and are prone to electrostatic problems.
Finally, Halon fire suppression systems are used in some high performance aircraft (e.g. F-16). Unfortunately, Halon has been banned from production because of its adverse impact on atmospheric ozone.
Another known system cools the ullage mixture to condense fuel vapors and to maintain the fuel tank ullage at a relatively low fuel-air ratio, significantly lower than the flammability limit.
All the above methods suffer from various drawbacks, such as weight, volume and cost penalties. The present disclosure is directed toward addressing at least one of these issues.